Urinary bladder

General Information

The bladder is a musculomembranous sac which acts as a reservoir for the urine. Its size, position, and relations vary according to the amount of fluid it contains, when it is empty or distended. In both conditions, the position of the bladder varies with the condition of the rectum; it is pushed upward and forward when the rectum is distended.

Parts and Relations

When hardened in situ, the empty bladder has the form of a flattened tetrahedron, with its vertex tilted forward. The empty bladder contains a fundus, vertex, a superior and an inferior surface. The fundus is triangular in shape and is directed inferior and posterior toward the rectum, from which it is separated by the rectovesical fascia, the seminal vesicle, and the terminal portions of the ductus deferens. The vertex is directed forward toward the upper part of the pubic symphysis. From it, the middle umbilical ligament is continued upward on the back of the anterior abdominal wall to the umbilicus. The peritoneum is carried by it from the vertex of the bladder on to the abdominal wall to form the middle umbilical fold. The superior surface is triangular and bound on either side by a lateral border which separates it from the inferior surface. It is also bound behind by a posterior border which is represented by a line joining the two ureters, which intervenes between it and the fundus. The lateral borders extend from the ureters to the vertex. From the peritoneum, it is carried to the walls of the pelvis. On either side of the bladder, the peritoneum shows a depression, named the paravesical fossa. The superior surface is directed upward. It is covered by peritoneum and is in relation with the sigmoid colon and some of the coils of the small intestine. The inferior surface is directed inferior and is uncovered by peritoneum. It can be divided into a posterior and two inferolateral surfaces. In the male, the posterior area is somewhat triangular; it rests upon the base of the prostate. From this area, the urethra emerges. The inferolateral portions of the inferior surface are directed downward and laterally. In front, it is separated from the pubic symphysis by a mass of fatty tissue which is named the retropubic pad. Behind, they are in contact with the fascia which covers the levator ani and obturator internus. In the female, the bladder is in relation posterior with the uterus and the upper part of the vagina. It is connected to the anterior side of cervix of the uterus, and the superior part of the anterior wall of the vagina by loose connective tissue. When the bladder is empty it is placed entirely within the pelvis, below the level of the obliterated internal iliac arteries. As it fills, the fundus is only slightly depressed. Its superior surface gradually rises into the abdominal cavity, carrying with it its peritoneal covering. It rounds off the posterior and lateral borders.
When the bladder is moderately full, it contains about half a liter and assumes an oval form. In this condition, it presents a posterosuperior, an anteroinferior, and two lateral surfaces, a fundus and a summit. The posterosuperior surface is directed upward and backward. It is covered by peritoneum. The anteroinferior surface is devoid of peritoneum. Below, it rests against the pubic bones. Above, it is in contact with the back of the anterior abdominal wall. The lower parts of the lateral surfaces are destitute of peritoneum and are in contact with the lateral walls of the pelvis. The fundus undergoes little alteration in position. The summit is directed upward and forward above the point of attachment of the middle umbilical ligament. The peritoneum which follows the ligament forms a pouch between the summit of the bladder and the anterior abdominal wall.

Ligaments

The bladder is connected to the pelvic wall by the parietal pelvic fascia. In front, this fascial attachment is strengthened by a few muscular fibers, pubovesical, which extends from the back of the pubic bones to the front of the bladder. Behind, other muscular fibers run from the fundus of the bladder to the sides of the rectum. The vertex of the bladder is joined to the umbilicus by the remains of the urachus which forms the middle umbilical ligament, a fibromuscular cord. This ligament is broad at its attachment to the bladder but narrows as it ascends. Anteriorly, there are three folds: the middle umbilical fold and two lateral umbilical folds.

Interior of the Bladder

The mucous membrane lining the bladder is loosely attached to the muscular coat. It appears wrinkled when the bladder is contracted. The folds are effaced when the bladder is distended. Over the trigon of the bladder, the mucous membrane is firmly bound to the muscular coat and is always smooth.

Layers of the Bladder

The bladder is composed of four layers: tunica serosa, tunica muscularis, tela submucosa, and the tunica mucosa.
The tunica serosa is derived from the peritoneum. It invests the superior surface and the upper parts of the lateral surfaces. It is reflected from these onto the abdominal and pelvic walls.
The tunica muscularis consists of three layers of unstriated muscular fibers: an external, middle, and an internal layer. The external layer is composed of fibers that have a longitudinal arrangement. These fibers arise from the posterior surface of the body of the pubis in both sexes. They pass up the inferior surface of the bladder, over its vertex, and then descend along its fundus to become attached to the prostate in the male and to the front of the vagina in the female. At the sides of the bladder, the fibers are arranged obliquely and intersect one another. This layer has been named the detrusor muscle. In the middle layer, the fibers are arranged in a circular manner. These fibers are very thinly and irregularly scattered on the body of the organ. Toward the lower part of the bladder, around the internal urethral orifice, they are disposed in a thick circular layer forming the internal urethral sphincter, which is continuous with the muscular fibers of the prostate. In the internal layer, the fibers have a general longitudinal arrangement. This layer is thin.
The tela submucosa consists of a layer of loose connective tissue tissue. It connects the tunica muscularis and the tunica mucosa.
The tunica mucosa is thin, smooth, and a pale rose color. It is continuous, above through the ureters with the lining membrane of the renal tubules, and below with that of the urethra. The loose texture of the tela submucosa layer allows the tunica mucosa to be thrown into rugae when the bladder is empty. Over the trigonum, the tunica mucosa is closely attached to the tunica muscularis and is not thrown into folds. It is covered in transitional epithelium. The epithelium varies depending on whether or not the bladder is distended.

Innervation

The nerves of the bladder are fine medullated fibers from the third and fourth sacral nerves and non-medullated fibers from the hypogastric plexus. They are connected with ganglia in the outer and submucous coats. They are distributed as non-medullated fibers to the muscular layer and epithelial lining of the bladder.

Blood Supply

The arteries supplying the bladder are the superior, middle, and inferior vesical. They are derived from the anterior trunk of the internal iliac. The obturator and inferior gluteal arteries also supply small visceral branches to the bladder. In the female, additional branches are derived from the uterine and vaginal arteries.

Venous Drainage

The veins form a complicated plexus on the inferior surface. They end in the internal iliac veins.